Using techniques developed to measure th relative rates of movement of several radiolabeled solutes of known molecular size across the epithelium of experimental animals, calculations can be made of its solute permeability. Adult animals subjected to high distending lung volumes markedly increase the solute permeability of the lung epithelium and studies are in progress to determine what effects a permeant lung epithelium has on the ability of the adult lung to maintain the separation of gas and liquid in the lung. In newborn animals initiation of breathing is accompanied by absorption of fetal lung liquid and increases in epithelial solute permeability. Newborn experiments are designed to study the relationship of epithelial solute permeability to biochemical lung maturity and the efficiency and completeness of lung liquid absorption.